When does my employer coverage actually end? (Oklahoma City)

I was told my coverage ends on my last day, but the letter says the end of the month. I do not want to be surprised by a bill. Did coverage match what HR said?

Thanks for writing this out. I went through something close to it last year, and it settled down once I kept a simple log of every call and form.

Dental and vision sometimes end on a different date from medical. Mine did.